1. Field of the Invention
The present invention relates generally to a semiconductor device and a method of fabricating the same, and more particularly to a technique for enhancing reliability and miniaturization of a ferroelectric random access memory.
2. Description of the Related Art
A ferroelectric random access memory (RAM) using a ferroelectric film in a cell capacitor has drawn attention as a next-generation non-volatile semiconductor memory. Jpn. Pat. Appln. KOKAI Publication No. 2000-307079, for instance, discloses a ferroelectric random access memory. This memory will now be described with reference to FIG. 1A. FIG. 1A is a cross-sectional view showing the structure of a prior-art ferroelectric random access memory.
As is shown in FIG. 1A, a memory cell of the ferroelectric random access memory, like a DRAM (Dynamic Random Access Memory), comprises a cell transistor 100 and a cell capacitor 200. A ferroelectric film is used as a capacitor insulating film 210 of the cell capacitor 200.
Jpn. Pat. Appln. KOKAI Publication No. 2001-257320 discloses a memory which consists of series connected memory cells each having a transistor having a source terminal and a drain terminal and a ferroelectric capacitor inbetween said two terminals, hereafter named “Series connected TC unit type ferroelectric RAM.”. This structure will now be described with reference to FIG. 1B. FIG. 1B is a cross-sectional view of a prior-art series connected TC unit type ferroelectric RAM.
As is shown in FIG. 1B, in the series connected TC unit type ferroelectric RAM, two cell capacitors share a capacitor lower electrode 220. Two capacitor upper electrodes 230 are provided over the lower electrode 220, with ferroelectric films 210 interposed. The lower electrode 220 is electrically connected to two cell transistors 100 via a contract plug 240.
In the prior-art series connected TC unit type ferroelectric RAM, however, a resistance between the capacitor lower electrode and the contact plug is high. Consequently, the reliability of the ferroelectric RAM lowers and the yield of manufacture deteriorates.
Conventionally, the use of tungsten or polysilicon as materials of contact plugs has been studied. However, these materials tend to be oxidized by high-temperature oxygen treatment for crystallizing a ferroelectric film of a cell capacitor, or high-temperature oxygen recover treatment for recovering a damage due to anisotropical etching. This leads to an increase in contact resistance between the capacitor lower electrode and the contact plug, and consequently to a decrease in yield.
A means for solving this problem is to increase a contact area between the capacitor lower electrode and the contact plug. From standpoint of miniaturization of memory cells, it is desirable, however, that the capacitor lower electrode and capacitor upper electrode have the same size. But, it is difficult to increase the size of the capacitor lower electrode. Besides, if the size of the contact plug is increased, as shown in FIG. 1C, miniaturization of memory cells becomes difficult. Thus, there is a problem that the miniaturization of memory cells and the decrease in contact resistance cannot be satisfied at the same time.